Lionel uses Pantone colors for their painting diagrams. To start the Pantone system is limited so an exact color match isn't possible to start. Factor in that paint manufacturers around the world use different types of paint bases. Depending on what factory painted it, there could be a range of drift off of the specified color. It is not surprising to see runs vary in color, but the question of how much variation is really the issue.
When using modeling paint, it also ages in the bottle and the color shifts. A 20 year old bottle of Scale Coat unopened will be a different color than an opened one.
As others have mentioned, PRR Tuscan Red did drift over the years and comparing photographs is not an accurate way to get the right color. Exposure, film type, lighting, and how it was printed doesn't help us as modelers at all. Add the challenge that color film has about a 25 year life span unless it was shot on Kodachrome which is closer to 50 years. The color shifts on actual film and eventually fades away after that initial life span.
My favorite example is a member of this forum who has the keystone herald off a B60B in his train room from when it was scrapped. It includes the entire body panel with it. Every conceivable color of Tuscan Red in this one 2'x2' steel plate.
Long answer to stating why it is very difficult to match colors in general unless original drift cards can be found. For the PRRT&HS, they have spent many, many years in discussion over "correct".
